The present invention relates to a fault localization system for use in optical fiber communication systems.
Optical fiber communication systems now in use utilize, as a repeating method, an optical-to-electric-to-optical conversion which involves converting an optical signal into an electric signal, amplifying and waveform-shaping the electric signal, and driving again a semiconductor laser with the amplified electric signal.
On the other hand, there has been proposed, as a new repeating system, an optical amplification method which directly amplifies an optical signal. The repeating system utilizing the optical amplification is advantageous over the conventional optical-to-electric-to optical conversion repeating system in that the repeating circuits used are simplified and that a wavelength-multiplex signal and a frequency-multiplex signal at a given transmission rate can be amplified collectively by one optical amplifier. Furthermore, since the amplifier gain is fixed regardless of the direction of incidence of light, optical signals in two ways can be amplified together by one optical amplifier.
The optical amplification is performed mainly by a method using a semiconductor laser and an optical fiber Raman amplification method utilizing stimulated Raman scattering which occurs in an optical fiber. Up to now results of many studies have been reported on amplification characteristics and noise characteristics of these optical amplification methods.
Concerning the direct optical amplification system it is now predicted theoretically that a maximum amplifier gain of 30 to 40 db is obtainable and that a signal can be transmitted, in the form of light, over 10,000 km without being affected by a background light noise caused by spontaneous scattered light, and hence the practicality of this as system as a future optical repeating system is becoming higher (IEEE. J. Lightwave Tech., Vol. LT-4, pp. 1328-1333 and IEEE. J. Quantum Electron, Vol. QE-17, pp. 919-935). When this optical repeating system is put into practical use, an optical cable having a repeater will be considered as a mere optical transmission line just like a non-repeating optical cable, and the combination of this system and an optical branching technique will make possible a multipoint communication in which a frequency (or wavelength) is allocated to each section between respctive points.
However, no proper fault monitor system has been proposed for use in such an optical fiber transmission system employing an optical amplifier.